If you add a 25mm (1") spacer to a 58mm offset wheel, the spacer width is subtracted, not added. 58 - 25 = 33mm offset.

And you aren't calculating the backspace, you are just calculating the equivalent offset.

A 10.5" wheel with a 58mm offset produces approximately an 8" backspace. If you add the 1" (25.4mm) spacer, the wheel will move outward 1", putting the back of the wheel equivalent to the position a 10.5" wheel with a 33mm offset (approx. 7" backspace) wheel would sit.

My 17x11 50mm wheels have a 4.0" frontspace, and the outer edge of the tire is just barely under the edge of the fender, looking straight down from above. The 10.5" 33mm wheel will have a frontspace of about 4.4", meaning that the outer edge of the tire will be slightly outside the fender.

Hold the end of the ruler against the outer edge of the tire, not the wheel.